1. Field of the Invention
The present invention relates to an optical amplifier for use in optical communication and optical information processing. Particularly, the present invention relates to an optical amplifier capable of maintaining amplified signal light level to a constant level at high precision.
2. Description of the Related Art
In the long distance optical communication system, it is common to adopt a system for inserting an optical amplifier into an optical fiber transmission system as means for extending a transmission length. In the optical distribution system for distributing a light signal to various locations, an optical amplifier has been widely used for compensating for a distribution loss.
Conventionally, as a light amplifier for directly amplifying a light signal, an optical fiber amplifier using a rare-earth doped optical fiber as an amplifying medium and a semiconductor optical amplifier using stimulated emission phenomenon within a semiconductor device have been well-known. At present, the optical fiber amplifier is commercially well-known.
If such an optical amplifier is used in, for example, an optical transmission equipment, it is required to maintain constant a light signal level in every part of the system for purposes of stabilizing transmission characteristics. Due to this, it is said to be necessary to provide a function of keeping constant the signal light output of the optical amplifier inserted into the transmission path.
Conventionally, as ways to control the signal light output power of the optical amplifier to be constant, there has been proposed a method for monitoring the entire output light power of the optical amplifier and controlling the power to be constant. Generally, however, amplified spontaneous emission as well as an amplified signal light is outputted from the optical amplifier. The ratio of the spontaneous emission to the entire light power varies according to the input light power. The optical amplifier output power monitoring method used in the above-stated control method cannot measure an output signal light level at high precision due to the influence of the spontaneous emission. Accordingly, even if the optical amplifier unit is controlled to make the output light level of the optical amplifier constant, it is difficult to maintain constant the signal light output level. As ways to reduce the influence of spontaneous emission, there has been proposed means for eliminating spontaneous emission using an optical filter. However, the optical filter cannot eliminate a component having the same wavelength as that of the signal light. Due to this, even with such an optical filter, it is impossible to completely avoid the influence of spontaneous emission.
Even in case no signal light is inputted into the optical amplifier unit, spontaneous emission is outputted from the optical amplifier unit and it is impossible to determine whether or not a signal light is present from the result of monitoring the output power of the optical amplifier. Due to this, it is necessary to add means for detecting whether or not a signal light is present to the input of the optical amplifier unit so as to operate the optical amplifier only in case the signal light is inputted. As a result, the number of optical components constituting the optical amplifier increases.